Passive immunization with anti-tumor antibodies has resulted in clinical improvement in a variety of neoplastic diseases. During the last funding period, studies were carried out to determine the role of Fc receptor mediated effector cell activation in the in vivo responses to variety of anti-tumor antibodies. Those studies indicated that the activation FcgammaR, FcRIII, was necessary for activity; genetic ablation of this receptor in both syngeneic and xenograft tumor models or mutation of the FcRIII binding site on the IgG1 Fc domain abrogated anti-tumor activity. Conversely, the inhibitory FcgammaR, FcRIIB, limited the potency of these antibodies against their tumor targets; deletion of FcRIIB enhanced anti-tumor activity by 10-100 fold in vivo. The contribution of effector cells and FcRs to the in vivo activity of anti-tumor antibodies will be further defined in the next funding period through the following three aims. Aim 1 will address the feasibility of constructing Fc mutants in anti-tumor antibodies to optimize activation FcR engagement while limiting engagement of the inhibitory FcRIIB molecule. Both murine and human IgG1 Fc mutants will be constructed and tested either in a syngeneic murine metastatic melanoma model or a B cell lymphoma xenograft model in which the murine FcRs have been replaced with their human counterparts. Aim 2 will explore the effect of modulating FcRIIB binding or expression on the activity of anti-tumor antibodies in vivo. Monoclonal antibodies capable of blocking either the mouse or human FcRIIB receptor will be used in syngeneic or FcR humanized, xenograft models, respectively, to determine their ability to act as adjuvants for specific anti-tumor antibodies. Biological modulators of FcRIIB expression, such as C5a, will also be tested for its ability to down-regulate FcRIIB expression on macrophages and thus enhance anti-tumor antibody activity. In Aim 3 the relative contribution of antigen signaling and FcR-triggered ADCC will be determined to the in vivo activity of anti-CD20 antibodies. Tumorigenic B cell lymphomas engineered to express a range of concentrations of CD20 surface molecules or to express CD20 variants crippled in their signaling domains will be evaluated for their ability to couple to FcR mediated effector pathways through anti-CD20 antibodies and be effective targets. These studies will further elucidate the mechanism of anti-tumor antibody action in vivo and suggest approaches for enhancing their activity.